How does the QuickSwitch work as a Voltage Converter? A side effect of using a FET is that the transistor incurs a voltage drop between the gate and the source, called the voltage threshold, Vt. This value is subject to the manufacturing process but is typically around 1 volt for a 5V system. So, a 5V signal is automatically dropped to 4V as it passes through the switch in normal operation. It is also known that the voltage drop across a diode is 0.7V. The diode can be inserted between the supply voltage, Vcc, and the QuickSwitch to cause it to have a maximum voltage of 4.3V instead of 5V. With the math shown above, the combination of a diode and the Vt drop, we arrive at a 3.3V maximum output voltage. The process takes place in either direction with no additional logic.

Note: Certain QuickSwitch devices such as QS3384 shown in the example, have an internal current path between Vcc and ground to provide current for the diode. Other QuickSwitch devices, such as QS3861 do not have this path and a path via a 10 kΩ resistor would need to be added. See page 4 of App Note AN-11 for further explanation.